JPH0194834A - High frequency probe for nmr and nmr signal measuring method - Google Patents

Abstract

PURPOSE:To facilitate impedance matching and to prevent the generation of waveform strain, by connecting a variable resistor means to the coupling coil magnetically coupled with a resonance circuit and selectively changing resonance frequency or a Q-value. CONSTITUTION:The resonance circuit 30 of a transmitting probe consists of a transmission coil 21 and a resonating condenser 27, and a coupling coil 1 is magnetically coupled with the transmission coil 21 of the resonance circuit 30. A variable resistor means 10 is constituted of a DC inhibiting condenser 4, pin diodes 2a, 2b and a resistor R and acts as the series resistor of the coupling coil 1. Bias voltage is applied to the pin diodes 2a, 2b through the resistor R to control the impedance of the variable resistor means 10.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a high frequency probe used in NMR (nuclear magnetic resonance) and an NMR signal measurement method, and in particular, the present invention relates to a high frequency probe used in NMR (nuclear magnetic resonance) and an NMR signal measurement method, in which impedance matching is easy and the high frequency magnetic field has no waveform distortion. The present invention relates to a method for measuring an NMR signal using a high frequency probe for NMR in which no signal is generated.

[Prior Art] FIG. 11 is a configuration diagram of a conventional high-frequency probe for NMR described in, for example, Japanese Patent Laid-Open No. 124854/1983.

In the figure, (21) is a transmitting coil for transmitting a high frequency magnetic field. (21a) and (21b) are input terminals provided at one end of the transmitting probe including the transmitting coil (21), to which a transmitter (not shown) is connected.

(22) is a receiving coil for receiving an NMR signal induced from the measurement object by a high frequency/magnetic field. (2
2a) and (22b) are output terminals provided at one end of a receiving probe including a receiving coil (22), to which a receiving preamplifier (not shown) is connected.

(25a) to (25d) are resonance capacitors inserted at multiple locations in the receiving coil (22), and at least one
One becomes a variable capacitor and is used for impedance matching.

(27a) and (27b) are resonance capacitors inserted at multiple locations in the transmitting coil (21), at least one of which serves as a variable capacitor and is used for impedance matching.

(28) is the resonance capacitor (27a) and (27b)
This is a switch means inserted in between, and is composed of bin diodes (28a) and (28b) connected in antiparallel.

(40) is an impedance means connected in parallel between both ends A2 of the resonance capacitor (25c), and includes bin diodes (40a) and (40b) connected in antiparallel, and these bin diodes (40a) and (40b). and an inductance element (40c) connected to the resonant capacitor (25c), forming a resonant circuit together with a resonant capacitor (25c).

Next, the construction of the high frequency probe for NMR shown in FIG. 11, that is, the conventional NMR signal measurement method using this probe will be explained.

First, input terminals (21a) and (21b) of the transmitting probe
) is applied with a high frequency pulse from a transmitter to make the bin diodes (28m) and (28b) conductive. As a result,
A resonant current flows through the transmitting coil (21) and a high frequency magnetic field is transmitted.

At this time, a part of the high-frequency magnetic field interlinks with the receiving coil (22) and tries to cause a resonant current to flow through the receiving coil (22), but such a resonant current affects the spatial uniformity of the high-frequency magnetic field. This causes brightness unevenness during NMR image formation.

However, since the bin diodes (40a) and (40b) are made conductive and a resonant circuit is formed by the resonant capacitor (25c) and the inductance element (40c), there is a high impedance between both terminals AB. Thereby, the resonance current that tends to flow into the receiving coil (22) is suppressed.

In this way, when a high frequency magnetic field is applied, an NMR signal is induced from the measurement object (not shown), a weak high frequency magnetic field is generated, and this NMR signal is received by the receiving coil (22). At this time, the bin diodes (40a) and (4
0b) is non-conducting, so the receiving coil (22) has N
A weak resonant current flows due to the MR signal (received signal). Also, a bin diode (28a) in the switch means (28)
and (28b) are non-conductive, so the transmitting coil (21
) is magnetically non-interfering with the receiving coil (22), and the Q value of the receiving coil (22) does not decrease.

Note that, prior to measuring the NMR signal, it is necessary to match the impedance of the transmitting probe to 50Ω, for example, but if the power of the high-frequency pulse applied to the input terminals (21a) and (21b) is small, the switching means (28) Since there is no conduction, high-power high-frequency pulses are applied to perform impedance matching.

[Problem to be Solved by the Invention 1] As described above, the conventional high-frequency probe for NMR and the NMR signal measurement method use a high-power high-frequency probe during impedance matching of the transmitting coil (21) to make the switch means (28) conductive. There was a problem that a pulse was required and it was not simple.Furthermore, since the switch means (28) was a non-linear element, there was a problem that waveform distortion occurred in the generated high-frequency magnetic field.

This invention was made in order to solve the above-mentioned problems, and its purpose is to obtain a high-frequency probe NMR signal measurement method for NMR in which impedance matching of a transmitting coil is easy and waveform distortion of a high-frequency magnetic field does not occur. do.

[Means for Solving the Problems] The high-frequency probe for NMR according to the present invention includes a coupling coil magnetically coupled to a resonant circuit for transmitting or receiving a high-frequency magnetic field, and a variable resistance means connected to the coupling coil.
A control terminal for controlling the impedance of this variable resistance means is provided.

Further, in the NMR signal measurement method according to the present invention, during the transmission period of the high frequency magnetic field, the first variable resistance means of the coupling coil magnetically coupled to the transmitting coil is set to high impedance, and the coupling coil magnetically coupled to the receiving coil is set to high impedance. The second variable resistance means is set to low impedance, and the NMR signal reception period is
- The first variable resistance means has a low impedance and the second variable resistance means has a high impedance.

[Function] In the high frequency probe for NMR of the present invention, the resonance frequency or Q value is selectively changed by changing the impedance of the variable resistance means.

Further, in the NMR signal measurement method of the present invention, ``the resonance state of the transmitting probe and the receiving probe is changed during transmission and reception to prevent magnetic interference between the respective probes.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings. 1st
The figure is a block diagram of a high frequency probe for NMR according to the present invention, and (21) is the same as described above. Further, (27) is a resonant capacitor corresponding to (27a) and (27b), and constitutes a resonant circuit (30) of the transmitting probe together with the transmitting coil (21).

Note that although the case of a transmitting probe is shown here, the high frequency probe may be a receiving probe.

Although not shown, the resonant circuit (30) is provided with a capacitively or magnetically coupled variable capacitance diode for impedance matching, and the input terminal (in the case of a receiving probe) is connected via the variable capacitance diode. output terminal) is provided.

(1) is a coupling coil magnetically coupled to the transmitting coil (21) of the resonant circuit (30). (2a) and (2b) are a pair of pin diodes connected in anti-series, and are connected to the coupling coil (1). R is a pin diode (2
It is a resistor for bias application connected to each terminal of a) and <2b), and each resistance value is set to 10Ω to 500Ω.

(4) is a DC blocking capacitor inserted between the coupling coil (1) and pin diodes (2a) and (2b), and the capacitance is set to about 0.01 μF to 0.1 μF. . In addition, this DC blocking capacitor (4) is
Together with the pin diodes (2a), (2b) and the resistor R, the variable resistance means (10) is configured to act as a series resistance of the coupling coil (1).

Incidentally, here, the variable resistance means (10) is connected to the coupling coil (1
), but a transmission line, such as a coaxial cable, having an electrical length that is an integral multiple of λ/2 (half wavelength) relative to the operating frequency may be interposed between the two. because,
This is because the impedance does not change even if a transmission line of λ/2 electrical length is inserted.

(5a) and (5b) are control terminals provided in the variable resistance means (10), which apply a bias voltage to the pin diodes (2a) and (2b) via the resistor R.

Next, the operation of the high frequency probe for NMR according to the present invention shown in FIG. 1 will be explained with reference to the resonance current characteristic diagram shown in FIG.

When a bias voltage is applied to the control terminals (5a) and (5b) to put the pin diodes (2a) and (2b) in a reverse bias or zero bias state, the pin diode (2a)
The resistance value of (2b) is a high impedance of several hundreds of Ω or more. Therefore, the coupling coil (1) does not affect the inductance value of the transmitting coil (21).Therefore, as shown by the solid line in Fig. 2, the resonant frequency f0
Resonant current ■. flows.

On the other hand, when the pin diodes (2a) and (2b) are forward biased, the coupling coil (1) is short-circuited with low impedance, so the inductance of the transmitting coil (21) increases due to mutual induction with the coupling coil (1). value decreases.

In this forward bias state, when a large forward bias current of about 100 (a) A is passed through the pin diodes (2a) and (2b), the resistance value of the pin diodes (2a) and (3b) becomes almost zero, causing resonance. The inductance of the circuit (30) is significantly reduced. Therefore, the resonant current characteristic becomes as shown by the broken line, and the resonant frequency f1 increases significantly from the resonant frequency r0 in the reverse bias state. The rate of increase M of the resonance frequency f1 at this time is M = [1/ (1-K 2), where K is the coupling coefficient between the coupling coil (1) and the transmitting coil (21).
]l/2.

Also, when a small forward bias current of about 0.4 mA is applied, the resistance value of the pin diodes (2a) and (2b) becomes 10
Since the current flows through the coupling coil (1), a non-negligible Joule loss occurs. For this reason,
The resonant current characteristic becomes like a dashed line, and the resonant frequency f2
As increases, the resonant current decreases and the resonant circuit (30)
The Q value of is significantly reduced.

By using the pin diodes (2a) and (2b) as variable resistance means in this way, the resonance state of the resonant circuit (30) can be changed, and magnetic coupling with other resonant circuits (not shown) can be reduced. can be reduced.

Figure 3 shows a pair of pin diodes (2
Instead of a) and (2b), four pin diodes (2
2 is a configuration diagram showing an embodiment in which a) to (2d) are configured as a bridge, and the operation is the same as that in FIG. 1. FIG.

FIG. 4 is a configuration diagram showing an embodiment in which the transmitting coil (21) is of a slotted tube type.
Coupling coil (1) at a position where the magnetic coupling with 1) is large.
are placed.

According to experiments, when the resonant frequency f0 of the transmitting coil (21) in Fig. 4 is 10 MHz, the pin diode (2a
) and (2b), the resonance frequency "1" was increased by 60011z by flowing a large forward bias current of about 100 mA.

Furthermore, by flowing a small forward bias current of about 1 mA, the non-resonant frequency f2 increased by 400 Hz, the Q value at the resonant frequency to <= 10 MHz> decreased significantly, and the resonant current ■ decreased to a negligible extent.

In this way, the pin diodes (2a) and (2b) are
It can be seen that by setting the III bias state, neither probe is magnetically coupled to other receiving probes. In reality, it has been found that it is easier to control by flowing a small forward bias current to reduce the Q value because the voltage applied to the control terminals (5a) and (5b) is low. In addition, as the pin diodes (2a) and (2b), 0840108 manufactured by Unitroad was used.

FIG. 5 is a configuration diagram showing an embodiment in which the transmitting coil (21) is of an orthogonal excitation type capable of transmitting a rotating magnetic field, and is shown by arrow C.
As shown in FIGS. and D, coupling coils (1) are arranged so as to be magnetically coupled in two directions of the high frequency magnetic field generated from the transmitting coil (21).

FIG. 6 is a configuration diagram showing an example in which a loop gap resonator type receiving coil (22) is arranged in place of the transmitting coil (21) and used as a receiving probe, and (11)
is a coil for impedance matching magnetically coupled to the receiving coil (22), (12) is a variable capacitance diode for impedance matching connected to the coil (11), (22)
^) is an output terminal corresponding to the above-mentioned (22a) and (22b), and (25) is a resonance capacitor corresponding to the above-mentioned (25a) to (25d). Also, the receiving coil (22)
and a resonance capacitor (25) constitute a resonance circuit (30A).

According to experiments, when the resonant frequency f0 of the receiving coil (22) in Fig. 6 is 25.5 MHz, the pin diode (
Forward bias current of 0.2m for 2a) and (2b)
By flowing only A, the Q value of the receiving probe was significantly reduced and the resonance current flow became negligible.

FIG. 7 is a configuration diagram showing an embodiment in which the coil (11) in FIG. 6 also serves as a coupling coil, and (4a)-(4
c) is a DC blocking capacitor, and (14) is a resistor for applying bias to the variable capacitance diode (12).

FIG. 8 also shows the transmitting coil (21) and receiving coil (2).
2) is a configuration diagram showing an example in which 2) is combined;
(1a) is the first coupling coil magnetically coupled to the transmitter coil (21), (1b) is magnetically coupled to the receiver coil (22).
The second coupling coil (10M) is the first coupling coil (
(1a) is a first variable resistance means connected to the second coupling coil (1b); (10b) is a second variable resistance means connected to the second coupling coil (1b);
(5c) and (5d) are control terminals provided in the second variable resistance means (10b).

Next, an embodiment of the NMR signal measuring method of the present invention using the NMR high frequency probe shown in FIG. 8 will be described with reference to the flowchart shown in FIG. 9 and the timing chart shown in FIG.

During the period when the transmitting coil (21) transmits a high-frequency magnetic field, the voltage v1 applied between the control terminals (5a) and (5b) is reverse biased (or zero biased), and the control terminals (5C) and (
The applied voltage v2 between 5d) and 5d) is set to a forward bias. As a result, the first variable resistance means (10a) becomes high impedance and the first coupling coil (1a) is opened, while the second
The variable resistance means (10b) has a low impedance (e.g. 1
0Ω), and the second coupling coil (1b) is short-circuited.

As a result, a resonant circuit (30) including a transmitting coil (21)
The resonant state of the receiver coil (22) remains unchanged, while the resonant frequency f of the resonant circuit (30A>) including the receiving coil (22) increases and the Q value decreases significantly (see FIG. 2).

At this time, the magnetic coupling between the transmitting coil (21) and the receiving coil (22) is almost negligible, and no resonant current flows through the receiving coil (22), so the spatial uniformity of the high-frequency magnetic field is improved, and the transmitting coil (21) ) behaves ideally. Further, by using a variable capacitance diode (12) similar to that shown in FIG. 6, impedance matching of the transmitting coil (21) can also be performed with low power.

Furthermore, since no distortion occurs in the transmitted waveform of the high-frequency magnetic field, no waveform correction is required.

In addition, by increasing the forward bias current, the second variable resistance means (10
If the impedance of b) is set to about 0.1a, Q
Although the value does not decrease and only the resonance frequency rl (see FIG. 2) increases, the magnetic coupling decreases as well.

During the period in which the NMR signal is received by the λ2j system receiving coil (22), the applied voltage (1) is made to be a forward bias, and the applied voltage (v2) is made to be a reverse bias. As a result, the first variable resistance means (10a) becomes a low impedance and the first coupling coil (1a) is short-circuited, while the second variable resistance means (tab) becomes a high impedance and the second coupling coil (1b) becomes a low impedance. ) is released. Therefore, the resonant frequency of the resonant circuit (30) increases to "1" (or the Q value decreases as it increases to f2), and the resonant circuit (30A) remains at the resonant frequency ".".

As a result, the magnetic coupling between the transmitting coil (21) and the receiving coil (22) is reduced, so the Q value of the receiving coil (22) does not decrease, and the receiving coil (22) operates ideally.

Also, impedance matching can be easily accomplished in the same way as in the first step.

[Effects of the Invention] As described above, according to the present invention, there is provided a coupling coil magnetically coupled to a resonant circuit for transmitting or receiving a high frequency magnetic field, a variable resistance means connected to the coupling coil, and a variable resistance means connected to the coupling coil. Since the device is equipped with a control terminal for controlling the impedance of the means and the resonant frequency or Q value is selectively changed, the impedance matching is easy and the high frequency probe for NMR can be used without waveform distortion.

Further, according to the present invention, during the transmission period of the high frequency magnetic field, the first variable resistance means on the transmitting coil side is set to high impedance, and the second variable resistance means on the receiving coil side is set to low impedance, and during the receiving period of the NMR signal, the first variable resistance means on the receiving coil side is set to low impedance. By setting the first variable resistance means to a low impedance and setting the second variable resistance means to a high impedance to change the resonance state and prevent magnetic interference between the probes during transmission and reception, impedance matching of the transmitting coil is achieved. This has the effect of providing an easy, high-performance NMR signal measurement method with a good signal-to-noise ratio.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram showing an embodiment of the high frequency probe for NMR of this invention, Fig. 2 is a resonant current characteristic diagram for explaining the operation of Fig. 1, and Figs. 3 to 8 are for NMR of this invention. 9 and 10 are flowcharts and timing charts showing one embodiment of the NMR signal measurement method of the present invention, and FIG. 11 is a diagram showing a conventional high frequency probe for NMR. It is a block diagram of a probe. It is a diagram. (1)...Coupling coil (1a)...First coupling coil (1b)...Second coupling coil (2a)-(2d)...Bin diode (5a)-(
5d)...Control terminal (10)...Variable resistance means (
10a)...First variable resistance means (10b)...Second variable resistance means (21)...Transmission coil (22)...Reception coil (30), (30^)...Resonance circuit In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (5)

[Claims] (1) A resonant circuit for transmitting or receiving a high-frequency magnetic field, a coupling coil magnetically coupled to this resonant circuit, a variable resistance means connected to this coupling coil, and a method for controlling the impedance of this variable resistance means. A high frequency probe for NMR equipped with a control terminal. (2) The high frequency probe for NMR according to claim 1, wherein the variable resistance means includes a pair of pin diodes connected in anti-series. (3) The first claim characterized in that the variable resistance means includes a plurality of pin diodes arranged in a bridge configuration.
High frequency probe for NMR described in section. (4) Any one of claims 1 to 3, characterized in that a transmission line having an electrical length that is an integral multiple of λ/2 is interposed between the coupling coil and the variable resistance means. A high frequency probe for NMR described in . (5) NM that transmits a high frequency magnetic field with a transmitting probe and receives an NMR signal from the measurement target with a receiving probe
In the R signal measurement method, a first coupling coil magnetically coupled to a transmitting coil of the transmitting probe, a second coupling coil magnetically coupled to a receiving coil of the receiving probe, and a first coupling coil connected to the first coupling coil. 1 variable resistance means and a second variable resistance means connected to the second coupling coil, and during the period of transmitting the high frequency magnetic field, the first variable resistance means is set to high impedance and the second variable resistance means is set to high impedance. A method for measuring an NMR signal, characterized in that the impedance of the first variable resistance means is set to low impedance and the second variable resistance means is set to high impedance during a period of receiving the NMR signal.